1. Field of the Invention
The present invention relates to a charge control device of a secondary battery used for driving an electronic appliance and the like.
2. Description of the Background Art
A rechargeable secondary battery is widely used as a power supply of a portable electronic appliance or an electrical appliance (hereinafter, referred to as a “portable appliance”) such as a notebook type personal computer (hereinafter, referred to as a “notebook computer”), a digital video camera, a digital still camera, a mobile telephone and the like. The secondary battery used for the portable appliance needs to have a small size, a light weight, a high capacity, a low memory effect, and a long life. Recently, a lithium ion secondary battery (hereinafter, referred to as a “lithium ion battery”) has been developed as a secondary battery which satisfies such demands.
Compared with other small sized secondary batteries such as a NiCad battery or a nickel hydrogen battery, the lithium ion battery has excellent characteristics such as high voltage, high energy density, no memory effect, long cycle life, and high storage capacity. Furthermore, for other characteristics such as high output, safety, and rapid charging capability, the lithium ion battery has similar performance to the NiCad battery or the nickel hydrogen battery.
A discharge capacity, which is measured as a difference in electrical charge between a fully charged state and a fully discharged state, is maximum at an initial stage of use, and the discharge capacity gradually decreases by repeating a charge and discharge of the secondary battery. The number of times of the charge and discharge of the secondary battery performed until the discharge capacity reaches a predetermined value is referred to as a cycle life. In other words, if the discharge capacity decreases to below the predetermined value, the secondary battery is considered to have reached an end of its life even though the secondary battery still can be practically used. The discharge capacity corresponds to a load capacity of the secondary battery per charge, and also corresponds to a usage period of the portable appliance driven by the secondary battery per charge.
From this standpoint, a product of the discharge capacity and the cycle life is determined as a total electrical capacity in which the secondary battery may be operable to function, or as a total operating period of the portable appliance driven by the secondary battery. In the present embodiment below, characteristics of the secondary battery, e.g., a cycle life, used for a portable appliance are described as an example. However, it is to be understood that the present invention is applicable to any portable or fixed electrical appliances driven by the secondary battery. Therefore, the electrical appliance driven by the secondary battery according to the present invention is generically referred to simply as an “appliance”.
Generally, a characteristic of the lithium ion battery is that when a charging voltage is high, a charging rate of the battery becomes higher, thus increasing the discharge capacity. In other words, the appliance can be used for a longer time per charge, while the cycle life becomes shorter. On the other hand, when the charging voltage is low, the charging rate becomes lower, thus decreasing the discharge capacity. In other words, the appliance is used for a shorter time per charge, while the cycle life becomes longer. Here, it is known that the total electrical capacity of the secondary battery and the total operating period of the portable appliance driven by the secondary battery can be both improved when the charging voltage is low rather than high. Therefore, there is a proportional relationship between the cycle life and the total electrical capacity of the secondary battery or the total usage period of the portable appliance driven by the secondary battery.
From this standpoint, various charge control devices have been developed in order to extend the cycle life of the secondary battery. In order to extend the cycle life, the Japanese Laid-Open Patent Publication No. 2002-51478 had proposed a charge control device allowing a user to select a charged level through an explicit instruction from the user.
However, since the charged level of the secondary battery significantly varies depending on the condition of use, it is very difficult to select an appropriate charged level in the aforementioned method. For example, when a notebook computer is mainly used for deskwork, mostly being connected with an AC adopter during the usage, a charge capacity of the secondary battery is preferably suppressed in order to extend the cycle life. On the other hand, the notebook computer is irregularly brought out to be used, away from home or the like, without being connected with the AC adopter, the charge capacity of the secondary battery needs to be temporarily increased in order to secure the operating period. For both conditions of use of the secondary battery described above, it is necessary, from a perspective of the user's convenience, that the user can instantly recognize a current condition of use of the secondary battery and rapidly change to a desired charged level (i.e., a predetermined charged level).
Furthermore, for realizing two contrary objects, i.e., (1) extending the operating period of the portable appliance driven by the secondary battery, and (2) extending the cycle life of the secondary battery, the charged level of the secondary battery must be accurately controlled. However, in a conventional control method based on a remaining capacity of the secondary battery, the charged level may not be controlled as desired since a precision for detecting the remaining capacity is reduced as characteristics of the secondary battery vary over time.